Tobacco leaves cannot be dehydrated and cured or dried by usual physical or mechanical curing or drying means to produce cured leaves of desired quality, because of the special physical structure and characteristics peculiar to their cells. Accordingly, unless the yellow coloring and the curing process is carried out with a proper adjustment of the dehydration speed and drying or curing temperature which does not destroy the cell of leaves, deterioration of the quality of the cured leaves will be the result.
Accordingly, what is important in drying or curing tobacco leaves is to prevent the cell from becoming excessively dried by controlling the humidity on the ambient temperature of the leaves in the curing process and the latent heat transferred to the cell so as not to destroy the cell.
The tobacco leaf curing process is usually divided for convenience into a first half period comprising the preheating stage for fermentation and the yellow coloring stage and a second half period comprising the leaf drying stage and the stem drying stage.
In the first period, saccharification of the ingredients and therefore the chemical and physiological change in the leaves proceeds for about 30 to 40 hours due to the action of the active enzyme in the leaves. During this period the temperature and water content of the leaves is maintained at a proper level.
During the first processing period, the green color of the leaf produced by the chlorophyll content vanishes and a yellow color resulting from the carochine content appears. This yellow color taken on by the leaf is peculiar to ordinary tobacco leaves. The fermentation, preheating, and the yellow coloring stage of the first half period can be carried out selectively and mechanically by conventional circulation drying methods (bulk curing), wherein wet hot air is circulated through the leaf chamber to produce the chemical change in the leaf ingredients.
In the second half period wherein the mesophyll setting and stem drying processes take place, the mesophyll and stem are dried and finished to dried tobacco leaves having a good quality, and a good aroma and smoking property. In the second period, however, an extremely elaborate and complicated adjustment is required and frequently failure in drying and accordingly deterioration in the quality of the leaves occurs, which leads to product and economic loss. The reason for this difficulty or failure is that although the latter half period consists only of the water from the mesophyll, the transition from the first half stage to the latter stage must be very carefully conducted. Thus, the process in the first period should be performed by maintaining such a proper upper temperature limit so as not to destroy the cell while causing the color of the leaves to change to yellow, and then the process should be slowly converted to the leaf drying or dehydration stage in the second half period with great care, without causing a drop in the leaf temperature due to the evaporation of moisture from the leaves.
Most of the failure in the past has been caused by the failure to provide a smooth transition from the first period to the latter period because the changing operations include a complicated temperature and humidity control.
According to the conventional methods of changing slowly from the curing process to the drying process, a "Standard Drying Operation Table" as shown in FIG. 24 has been used. In the conventional method, temperature adjustment was conducted by always checking the difference between the dry-bulb temperature and the wet-bulb temperature of the curing barn while referring to the operation table.
With respect to the relationship between the dry-bulb and the wet-bulb temperatures during the drying process, the following should be noted: The dry-bulb temperature rises successively insteps while the wet-bulb temperature is linerally maintained from the beginning to the end, and up to about 38.degree.C. and then substantially stays there. In the examples which failed, it was found that the wet-bulb temperature fluctuated in wave-like fashion.
It was noted that the water evaporation speed on the surface of the cloth piece covering the wet-bulb in the drying chamber was quite similar to that on the surface of the tobacco leaves. When both evaporation speeds are substantially equivalent, the temperature of both surfaces are quite similar. Accordingly, the temperature of the leaves can be determined by observing the drying degree of the covering cloth of the wet-bulb, i.e., the wet-bulb temperature. In short, to adjust the wet-bulb temperature at about 38.degree.C. is to set the water evaporation speed for the leaf at an evaporation speed level of about 38.degree.C.
Accordingly, in view of the fact that it is necessary to stabilize the wet-bulb temperature at about 38.degree.C. in order to succeed in effectively drying the leaves, one of the purposes of the present invention is to provide a tobacco leaf drying system which dries tobacco leaves perfectly and completely at a natural temperature rise starting from about 43.degree.C. of the dry-bulb temperature, and stabilizing the wet-bulb temperature at about 38.degree.C., also in the second half period. At the beginning of the mesophyll drying stage where moisture in the leaf evaporates actively, most of the heat supplied by the burner is used as latent heat for the evaporation of the water of the leaf and accordingly does not serve to raise the dry-bulb temperature. But in the later part of the mesophyll setting process where the leaf becomes almost dried, the evaporation of the moisture drops, and the amount of latent heat used becomes less. Accordingly a larger amount of the burner heat is used spontaneously to raise the temperature of the dry-bulb. This is called the spontaneous temperature rise of the dry-bulb temperature.
In the mesophyll drying and stem drying stage it is necessary to control the intake and exhaust of the circulating air in the drying chamber (the bulk curing barn) while keeping the wet-bulb temperature set at about 38.degree.C. The air control procedure is an important step in the tobacco leaf drying process, along with the control of the wet-bulb temperature.
Drying of the leaf, i.e., evaporation of the water in the leaf is possible so long as the saturated vapor pressure of the water contained in the leaf is greater than the partial pressure of the vapor contained in the ambient air in the chamber. The greater the differences between said two pressures, the faster is the drying speed. On the other hand because the latent heat is removed from the leaf due to the evaporation of the water, the temperature of the leaf lowers during the drying process.
In an air circulation drying method, the humidity content of the air having once passed through the layers of the leaf, approaches a saturated condition. Accordingly, the air should be reheated to increase the moisture containing capacity of the air. Thus, if only the dehydration of the leaf is desired, all that is necessary is to replace all of the water-containing air which has passed through the tobacco leaf layers by outdoor air. However, in the tobacco leaf drying process, the tobacco leaf should be gradually dried and set while the dehydration velocity and leaf is controlled so that the leaf ingredients change gradually over a long period of time. Accordingly the amount of intake and exhaust air should be maintained within about 5 to 30% of the total circulated air. Thus it becomes necessary as part of the drying process that most of the humid air having passed through the leaf must be reheated during its recirculation and sent into the drying chamber as a reheated hot humid air. This increases the thermal efficiency of the drying system and produces a good economic result.
Accordingly, an object of this invention is to provide a tobacco leaf drying system having an air-inlet opening and closing device which is operated independently of the wet-bulb temperature control device, and in operation, gradually starts to open the inlet when the mesophyll drying stage begins, and closes the inlet by the end of the stem drying stage. This ventilates the humid air having passed through the leaf layers in amounts almost equal to the outdoor air introduced into the chamber, so as to slowly change the mixing ratio of the fresh air taken in and, thereby maintaining the chamber humidity at a proper level while advancing a successful drying operation.
Another object of the present invention is to provide a tobacco leaf drying system which can simplify the tobacco leaf harvesting and handling procedures, such as packing the leaves at the cultivation site, transporting the leaves from the cultivation site to the site of the drying system, feeding the leaves into the chamber without damaging the leaves, drying the leaves as they are harvested, and packing the leaves, thereby greatly reducing the labor costs of the present system when compared with conventional harvesting, handling and drying processes.
A further object of the present invention is to provide a tobacco leaf hanging means used in the drying chamber wherein means are provided for suspending the leaves in the chamber from clamps attached to the bases of the leaves.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.